Biosynthesis of human polyunsaturated fatty acids (hereinafter, “PUFA”) occurs for two representative series, the ω3 and ω6 series (where ω represents the number of the carbon atom having the first double bond, counting from the methyl group end of the fatty acid), and in the case of ω6 fatty acids, for example, linoleic acid (18:2 ω6) is converted to γ-linolenic acid (18:3 ω6), dihomo-γ-linolenic acid (20:3 ω6), arachidonic acid (20:4 ω6) and 4,7,10,13,16-docosapentaenoic acid (22:5 ω6), by repeated desaturation and carbon chain elongation.
Similarly, in the case of ω3 fatty acids, α-linolenic acid (18:3 ω3) is converted to eicosapentaenoic acid (20:5 ω3), 7,10,13,16,19-docosapentaenoic acid (22:5 ω3) (docosapentaenoic acid) and 4,7,10,13,16,19-docosahexaenoic acid (22:6 ω3) (docosahexaenoic acid), by repeated desaturation and carbon chain elongation. The ω3 PUFAs eicosapentaenoic acid (hereinafter, “EPA”) and docosahexaenoic acid (hereinafter, “DHA”) in particular are known to have numerous physiological functions including prophylactic effects against adult diseases such as atherosclerosis and thrombosis or anticancer effects, as well as learning reinforcement effects, and various attempts have been made to utilize them in pharmaceuticals and food for specified health uses. However, PUFAs other than ω3 types (such as ω6 and ω9) have recently also been the subject of attention.
Arachidonic acid constitutes approximately 10% of the fatty acids composing vital organs such as the blood and liver (for example, the fatty acid compositional ratio of the phospholipids in human blood is 11% arachidonic acid, 1% eicosapentaenoic acid, 3% docosahexaenoic acid), and as a major structural component of cell membranes, it contributes to modulating membrane fluidity and performs various metabolic functions, while also playing an important role as a direct precursor of prostaglandins. Recently the roles of arachidonic acid as a nursing infant nutrient and as a constituent fatty acid of endogenous cannabinoids which exhibit neuroactivating effects (2-arachidonoyl monoglycerol, anandamide) have been noted. Normally, ingestion of linoleic acid-rich foods leads to their conversion to arachidonic acid, but since the functions of the enzymes involved in its biosynthesis are reduced in life-style related disease patients and preliminary conditions as well as in infants and the elderly, such individuals tend to be deficient in arachidonic acid; it has therefore been desirable to provide means for its direct ingestion in the form of a constituent fatty acid of fats or oils (triglycerides).
Although fish oils are abundant sources of ω3 PUFAs such as EPA and DHA, ω6 PUFAs such as γ-linolenic acid, dihomo-γ-linolenic acid, arachidonic acid and 4,7,10,13,16-docosapentaenoic acid (22:5 ω6) are virtually unobtainable from traditional fat or oil sources, and therefore fats and/or oils comprising PUFAs as constituent fatty acids (hereinafter referred to as “PUFA-containing fats and/or oils”) obtained by fermentation of microorganisms are most commonly used at the current time. For example, methods have been proposed for obtaining fats and/or oils comprising arachidonic acid as a constituent fatty acid (hereinafter referred to as “arachidonic acid-containing fats and/or oils”) by culturing of various microorganisms capable of producing arachidonic acid-containing fats and/or oils.
It is known that fats and oils having a high proportion of arachidonic acid constituting the fatty acid portion (hereinafter referred to as “arachidonic acid-rich fats and/or oils”) can be obtained by using microorganisms belonging to the genus Mortierella (Japanese Unexamined Patent Publication SHO No. 63-44891, Japanese Unexamined Patent Publication SHO No. 63-12290). In recent years, one of the essential uses of arachidonic acid is in the field of nursing infant nutrition, for example, and specifically involves the use of arachidonic acid-containing fats and/or oils obtained by fermentation in infant formula. New effects of arachidonic acid-containing fats and/or oils have also been demonstrated (Japanese Unexamined Patent Publication No. 2003-48831: Composition with prophylactic or ameliorative effect on symptoms and conditions associated with brain function impairment), and these are expected to be in high demand in the future.
Fats and/or oils obtained by culturing of Mortierella microorganisms consist largely of triglycerides (approximately 70% or greater) and phospholipids. The edible fats and/or oils are in the form of triglycerides, and for the purpose of the use described above, the original fats and/or oils produced by the cells (fats and oils obtained by extraction from cells, known as “crude oils”) are extracted from the cell biomass resulting from culturing of the microorganisms, and then the crude oils are subjected to edible fat/oil refining steps (degumming, deoxidation, deodorization and decolorizing) to obtain refined fats and/or oils without the phospholipids.
Since PUFA- containing fats and/or oils obtained by culturing of Mortierella microorganisms accumulate in mycelia, culturing must be carried out to a higher concentration to increase the yield of the PUFA-containing fats and/or oils per culture, for higher economical optimization of the fat/oil production. The PUFA-containing fat and/or oil yield per culture is the product of the cell or mycelial concentration and the PUFA-containing fat/oil content per mycelia, and it is therefore necessary to increase both the cell concentration and the PUFA-containing fat/oil content per culture. The cell concentration can be increased by raising the concentration of the nitrogen source in the culture medium which is normally converted to cell components. The PUFA-containing fat/oil content per mycelia can only be increased by satisfactorily controlling the cellular form and by carrying out the fermentation in the presence of adequate oxygen. Methods reported for controlling the cellular form include optimization of the medium salt composition (Japanese Domestic Re-publication No. 98/029558), while methods of supplying oxygen include pressurized culturing methods and oxygen enriched aerobic culturing methods (Japanese Unexamined Patent Publication HEI No. 06-153970).
Attempts to improve not only the culturing procedure but also the post-culturing cell recovery procedure have been reported as well. For example, one reported method involves acquiring a microbial biomass (20-75% moisture content) and granulating it into granular particles while maintaining the moisture content, and then drying it to a moisture content of below 20%, whereby the granulation facilitates not only drying but also extraction of the target compound (WO97/36996). This publication teaches that an extrusion method is preferred for molding into granular particles, but ordinary extrusion methods do not alter the moisture content in general.
The granular particles are dried by, for example, spray drying, fluidized bed drying, lyophilization drying, belt drying or vacuum drying. Another known method is one in which a culture solution of a Mortierella filamentous fungus is filtered to collect the cells, which are then dried and disrupted, and the fats and/or oils are extracted using an organic solvent (CN1323904A), while Yamada et al. have reported a disruption method using a ball mill (“Industrial applications of single cell oils”, edited by D. J. Kyle and C. Ratledge, AOCS press (1992) p. 118-138). Thus, although various different cell recovery methods have been published, the drying is invariably accomplished by a single step of conventional drying, whereas no development of using novel driers or using multiple conventional driers has been disclosed. Moreover, no dried microbial biomass processing method has been described.
Despite the fact that the cell recovery procedure is extremely important from the standpoint of loss or reduction of the microbial fats and/or oils and of microbial fat/oil quality in the microbial fat/oil production, virtually no publications can be found currently which relate to such process development.
Drying processes can be generally classified into three steps or periods (“Shokuhin Kogaku Kiso Koza (6) Concentration and drying”, R. Matsuno et al., Korin Press (1988), Chap. 5). First, if the material has an adequate moisture content, evaporation of water from the material is considered to be equivalent to evaporation of water from the water droplet surfaces, and the material temperature will shift toward the wet-bulb temperature during a period known as the pre-heating period. After the material has reached the wet-bulb temperature, the influx heat quantity from the air is completely consumed by moisture evaporation, and therefore the moisture content of the material decreases in direct proportion to time. The period of constant drying rate is referred to as the constant drying rate period. With further drying, migration of water inside the material becomes the rate-limiting factor, such that the moisture evaporation rate decreases and the moisture content reaches equilibrium with the dry air, eventually causing the drying to cease. This period is known as the falling drying rate period.
Practical drying methods may be largely divided into convection heating methods, conduction heating methods and radiation heating methods. The known radiation heating methods include infrared radiation methods, but such methods are not commonly employed for food processing involving large-scale bulk treatment, and instead convection and conduction heat methods are more widely used.
A convection heating type drier supplies hot air to rapidly remove evaporated moisture from the raw material vicinity, and thus powerfully promotes moisture evaporation; it is therefore an effective means for achieving massive moisture content reduction. On the other hand, however, the large hot air supply causes scattering of the dried material powder and raises the energy costs for the fans, while raw materials with high moisture contents lead to problems such as clumping due to adhesion among the materials, and reduced hot air contact area.
A conduction heating type drier can achieve high heat efficiency with virtually no air flow, and therefore blowing energy costs and scattering of raw material dust can be vastly reduced. On the other hand, however, heating occurs by heat conduction alone and thus it has been difficult to accomplish drying to a low moisture content.
Japanese Unexamined Patent Publication SHO No. 63-44891
Japanese Unexamined Patent Publication SHO No. 63-12290
Japanese Unexamined Patent Publication No. 2003-48831
Japanese Unexamined Patent Publication HEI No. 06-153970
Japanese Domestic Re-publication No. 98/029558
WO97/36996
CN1323904A
Industrial applications of single cell oils, edited by D. J. Kyle and C. Ratledge, AOCS press (1992) p. 118-138
Shokuhin Kogaku Kiso Koza (6) Concentration and drying”, R. Matsuno et al., Korin Press (1988), Chap. 5